Democratic candidate for U.S. Senate, Michelle Nunn speaks to her supporters after her primary win was announced at an election-night watch party Tuesday, May 20, 2014, in Atlanta.

Associated Press

Georgia Democrat Michelle Nunn had one of the best fundraising quarters of any candidate in this election cycle’s key Senate races, pulling in $3.5 million over the last three months, according to new data.

That’s nearly a million more than Republican businessman David Perdue, and over $1.5 million more than Republican Rep. Jack Kingston, both of whom are vying for the GOP nomination and still face a costly few days before their runoff.

Ms. Nunn’s latest haul—outdone only by Democrat Alison Lundergan Grimes in Kentucky, who raised $4 million—puts her in a solid position as she heads into the general election. Her cash reserves now stand at $2.3 million, while her challengers have far less in the bank: $1.2 million for Mr. Kingston, and $780,000 for Mr. Perdue. That means that whoever wins the July 22 runoff will need to scramble to catch up to Ms. Nunn.

The latest numbers were released late Tuesday and Wednesday by the candidates’ campaigns. Senate candidates were required to file their second-quarter fundraising reports with the Federal Election Commission at midnight on Tuesday, but since they postmark their reports, the filings will not be available on the FEC website for a few more days.

An early review of fundraising reports in seven of the key Senate races found that while on aggregate Democrats still held an advantage over their opponents in those races, it had diminished considerably since the first quarter. The same is true in the Georgia race: While Ms. Nunn’s fundraising stands far ahead of her challengers’, her cash-on-hand advantage over both Republicans has decreased since the first quarter. That’s despite the fact that both Republicans’ coffers have been significantly depleted as they battle to win the GOP nomination.

Recent polling shows that Ms. Nunn leads in a matchup with both Republicans. She held a three-point advantage over Mr. Kingston in a Public Policy Polling survey held July 11-13—within the poll’s margin of error—and a seven-point lead over Mr. Perdue. The survey also found Mr. Kingston with a six-point edge over his GOP opponent when respondents were asked whom they would vote for in the Republican runoff. The candidates are running to replace retiring Sen. Saxby Chambliss, a Republican.

In other Senate races:

MICHIGAN: In Michigan, Democratic Rep. Gary Peters raised $1.96 million in the second quarter of the year. That number is considerably less than Republican Terri Lynn Land’s $3.35 million—but, her spokeswoman noted, $1.2 million of that amount was loaned to the campaign by Ms. Land. That means the amounts each candidate raised are roughly similar.

Ms. Land now has $5.3 million in the bank. Mr. Peters has not yet disclosed his cash-on-hand figures, making it difficult to compare the candidates’ financial positions. Both are vying to replace retiring Sen. Carl Levin (D., Mich.).

WEST VIRGINIA: Democrat Natalie Tennant in West Virginia lagged slightly behind Republican Rep. Shelley Moore Capito last quarter, raising about $500,000 less than the incumbent. Ms. Capito has $5 million in the bank, while Ms. Tennant has not released the amount of her cash reserves. A fundraising advantage is not unusual for candidates already serving in Congress—Ms. Capito can use funds raised for her re-election race in the House. The race’s winner will replace retiring Sen. Jay Rockefeller, a Democrat.

The race has drawn high-profile politicians to West Virginia this week. Wisconsin Rep. Paul Ryan campaigned in Charleston with Ms. Capito, and Massachusetts Sen. Elizabeth Warren—the darling of progressive Democrats—rallied with Ms. Tennant this week. Ms. Tennant’s campaign said it expected to get a fundraising bump from Ms. Warren’s appearance.

IOWA: In Iowa, Democratic Rep. Bruce Braley and Republican state Sen. Joni Ernst are neck-and-neck—both in polling and in fundraising numbers. Ms. Ernst raised $1.8 million in the second quarter, just a hair more than Mr. Braley’s $1.7 million. The Democrat has a hefty cash advantage–$2.7 million to Ms. Ernst’s $1 million—but, like Ms. Capito, entered the race with a fundraising edge since he is currently a member of Congress.

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